How should I rig this scissor mechanism with different pivot points?

Question

I would like to use bones to rig the following mechanism but am unsure how and any advice is appreciated.

I have already tried this:

However, the bones are crossing over the "limit" on the top and bottom.

Specifically, I want Bone.002 to not cross the line marked by the dotted red line. And I want Bone to not go further down than Bone.001 (Bone.003 is me experimenting).

The problem is shown in the yellow circle. I want the same thing for the top and the bottom just thought I'd show 2 variations to hopefully make it more clear.

I am moving everything using Bone.004

If anyone has an alternative solution with a different bone setup that would also be great!

Answer 1

You can do this using IK with stretch and rotation enabled.

The scissor arm, parented to the "bottom" bone, is involved in a 2-bone IK chain, targeting a bone parented to the "top" bone; the second bone in the chain, ikstretch, is non-deforming. The IK has rotation and stretch enabled, and ikstretch has stretch set to 1.0 in its IK settings. Ikstretch and iktarget have the exact same rest axes.

When the top or bottom bones change elevation, the scissor rotates only to the level of the top bone, no less and no more, because there is no other way for the second bone to match the rotation and position of the IK target.

Note that stretch IK will never cause ikstretch to scale backwards. This can be useful in some situations (although not in this example); typically, if its not the behavior you want, you can just offset the location of the ik target so that it never has to scale backwards.

And of course, the behavior can't work if top and bottom bones are further apart than the length of scissor-- there's no solution in that case.

For the other arm, simply duplicate and mirror scissor, ikstretch, and iktarget.